1. Field of the Invention
The invention relates to the photoelectrochemical oxidation of thallium(I) to thallium(III) in solutions, and to the use of the thallium(III) in oxidation reactions with organic compounds.
2. Description of the Art
Thallium(III) has been found to be a very useful chemical oxidizing agent, having a reduction potential of 1.247 volts (versus the normal hydrogen electrode) for Reaction 1. EQU T1.sup.+3 +2e.sup.- .fwdarw.T1.sup.+ (1)
Use has been made of thallium(III) in a variety of organic synthesis schemes.
Organic compounds which can be reacted with thallium(III) include those which have an index of hydrogen deficiency greater than zero. This index is described by J. B. Hendrickson, D. J. Cram and G. S. Hammond, Organic Chemistry, Third Edition, McGraw-Hill, Inc., 1970, at pages 72-73 and 82-83, as the number of pairs of hydrogen atoms which must be removed from a saturated alkane to give the empirical formula of a subject compound. For a hydrocarbon, then, the index represents the total of the rings and multiple bonds in a molecule. For compounds containing heteroatoms, the following principles can be used to make the index application: (1) oxygen and sulfur atoms do not change the index; (2) each halogen atom is equivalent to one-half of a hydrogen atom pair; and (3) each nitrogen atom requires that the "reference" saturated alkane be considered as having one extra hydrogen atom (i.e., a formula of C.sub.n H.sub.2n+3).
A paper by Grinstead in Journal of Organic Chemistry, Vol. 26, pages 238-240 (1961), reported the oxidation of the olefins ethylene and 2-hexene by aqueous acidic solutions of thallium(III) to form carbonyl compounds and glycols or their esters. U.S. Pat. No. 3,048,636 to Grinstead also deals with this oxidation.
Reaction mechanism and kinetics for the oxidation of olefins by thallium(III) have been discussed in some detail by P. M. Henry in "Oxidizing Olefins by Pd(II) and Tl(III)," Homogeneous Catalysis, ACS Advances in Chemistry Series, Vol. 70, pages 126-154 (1968). The mechanism proposed for the oxidation, exemplified by that for ethylene, is as shown in Equations 2 and 3. ##STR1## which explains the experimentally observed product mixture.
By utilizing weaker solvating media for the thallium(III), Kruse and Bednarski reported in Journal of Organic Chemistry, Vol. 36, pages 1154 and 1155 (1971), that the oxidation of olefins can be halted at an intermediate epoxide stage. For example, thallium triacetate, in a solvent composed of tetrahydrofuran, water and acetic acid (70, 20 and 10 percent by volume, respectively), oxidized propylene to form a mixture of 72 percent propylene oxide, 16 percent acetone and 12 percent 1-acetoxy-2-propanol. This reaction is the subject of U.S. Pat. No. 3,641,067 to Kruse.
In addition to the oxidation of olefins, many other organic reactions occur with thallium. A review by R. J. Ouellette, "Oxidation by Thallium(III)," Chapter 3 of Oxidation in Organic Chemistry, Part B, W. S. Trahanovsky, Ed., Academic Press, 1973, discusses work which has been done in the oxidation of steroids, oxidative rearrangement of chalcones, oxidative cleavage of cycloalkanes, oxidation of carbonyl compounds, oxidation of phenols, and reactions with silanes. Other interesting uses for thallium(III) include the conversion of benzene to phenol, wherein the reaction occurs as in Equations 4 and 5. ##STR2## It is also possible to use a similar scheme to produce aniline, as shown by Equation 6. ##STR3## By using alkaline sulfides, thiophenols can be produced in a similar manner; haloaromatics can be prepared utilizing halide ions.
Unfortunately, however, regeneration of the thallium(III) from the thallium(I) formed in previously described reactions has presented a difficult problem. The thallic/thallous reduction potential is slightly positive of the oxygen/water couple (1.229 volts) of Equation 7, EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (7)
so that regeneration with molecular oxygen is not feasible. Attempts have been made to shift the reduction potential of the thallic/thallous couple (e.g., to 0.783 volts in 1 Formal hydrochloric acid), but such a shift is accompanied by a loss of oxidizing power for the thallium(III) species.
Chemical oxidation of thallous is, of course, possible with the very powerful agents such as chlorine gas and aqua regia, but these materials are objectionable as being somewhat difficult to handle (requiring expensive low-corrosion equipment), and cause the accumulation of undesirable materials in the system. A variety of methods for chemically oxidizing thallium(I) to thallium(III) with less objectionable materials have been developed.
Hirose et al., in U.S. Pat. No. 3,399,956, report a system for oxidizing thallium with oxygen, which involves an acidic aqueous medium containing chloride or bromide and an ion of a "redox metal" such as copper or iron. The metal ion is described as an "electron acceptor" when thallium(I) is converted to thallium(III).
In U.S. Pat. No. 3,479,262, MacLean et al. describe an olefin oxidation process which includes a noble metal catalyzed reoxidation of thallium by electrochemically generated cerium(IV). Following Equations 8 and 9 describe anodic and cathodic electrode reactions, respectively, while Equation 10 shows the thallium reaction. ##STR4##
Also shown by MacLean et al. is the direct electrochemical generation of thallium(III) from thallium(I) in a divided cell, wherein thallium is not permitted to migrate into the cathode compartment, thereby avoiding deposition of thallium metal at the cathode. The electrochemical reactions are demonstrated for sulfuric acid-containing thallium solutions.
Other electrochemical systems for oxidizing thallium are shown in U.S. Pat. No. 3,486,992 to Frye and in U.S. Pat. No. 3,759,804 to LeBris et al.
U.S. Pat. No. 4,031,196 to Leonard is concerned with the regeneration of thallium(III) which has been used in the form of its isobutyrate for the oxidation of unsaturated organic compounds. The rather complex procedure includes air oxidation of an alkaline solution of thallium(I) isobutyrate at elevated temperatures, to produce a slurry of thallium(III) oxide which is removed. The remaining solution is treated with carbon dioxide, producing isobutyric acid which is extracted with a solvent. Isobutyric acid then dissolves the thallium(III) oxide, forming the initial oxidizing reagent.
Methods have been developed for chemically oxidizing thallium(I) in the presence of noble metals. These methods include those of Brill, as in U.S. Pat. No. 4,115,420 (oxidation with molecular oxygen in strongly acidic solution) and 4,115,421 (oxidation using an organic hydroperoxide). Rizkalla, in U.S. Pat. No. 4,058,542, oxidized thallium(I) using molecular oxygen in the presence of both a Group VIII metal and a heterocyclic tertiary amine "promoter," obtaining a higher product yield than that of Brill. A similar procedure is reported by Johnson, who used an alkali metal compound as promoter in U.S. Pat. No. 4,113,756 and an alkyl ammonium salt as a promoter in U.S. Pat. No. 4,192,814.
Thallium(III) has been generated by reaction with a perorganic acid (e.g., a mixture of acetic acid and hydrogen peroxide) in the presence of a manganese or ruthenium compound promoter, as shown by Walker in U.S. Pat. Nos. 4,135,051 and 4,226,790.
The described methods each suffer from one or more of the following disadvantages: (1) the requirement for relatively expensive chemical reagents which are irreversibly consumed during thallium oxidation; (2) the need for a separate thallium oxidation facility external to the equipment for organic compound oxidation by thallium(III); (3) gradual accumulation in the thallium solution of undesired by-products from the thallium oxidation reaction; and (4) a significant expenditure of thermal or electrical energy to accomplish thallium oxidation.
Due to the recent substantial increases in the cost of energy, considerable interest and research has been generated for the utilization of alternative energy sources, including the harnessing of solar radiation. An area which has received attention in connection with oxidation-reduction chemical reactions is that of photoelectrochemistry, wherein the interaction of photons with a suitable photoelectrode creates a flow of current in an electrochemical cell. Much of the reported efforts in this area has dealt with various forms of solar energy storage, e.g., the photoelectrolysis of water to form storable hydrogen, as in U.S. Pat. Nos. 3,925,212 to Tchernev; 4,011,149 and 4,090,933 to Nozik; 4,100,051 to Kilby; and 4,144,147 to Jarrett et al.
Other solar energy storage schemes include that of McKinzie et al. in U.S. Pat. No. 4,128,704 where, in one embodiment, water is dissociated into molecular oxygen and hydrogen ions at a photoanode under ultraviolet light, and a corresponding cathode reaction reduces cupric ions to metallic copper. The energy can be subsequently recovered by connecting an electrical load between the metallic copper electrode and a "reducing electrode" to which oxygen is supplied, and immersing both electrodes in an acid solution.
Another application for photoelectrochemistry which has had considerably less research attention is that of utilizing solar energy to conduct preparative chemical reactions. Many of the reported efforts in this field have been concerned with photocatalysis, wherein the activation energy to initiate an otherwise exoergic reaction is supplied photoelectrochemically. Several of these reactions have been summarized in a review by Bard, "Photoelectrochemistry," in Science, Vol. 207, pages 139-144 (1980) (which is incorporated herein by this reference), including the photo-Kolbe reaction, wherein decarboxylation proceeds according to Equation 11 in a photoelectrochemical cell. EQU 2RCOOH.fwdarw.R--R+2CO.sub.2 +2H.sup.+ +2e.sup.- (11)
It is also possible to supply the required energy for endoergic chemical synthesis reactions, using a photoelectrochemical cell. This technique, which can be called "photoelectrosynthesis," has been utilized to produce amino acids from the reactants methane, ammonia, and water, as reported by Reiche and Bard in Journal of the American Chemical Society, Vol. 101, pages 3127-3128 (1979). Another paper by Frank and Bard in Journal of the American Chemical Society, Vol. 99, pages 4667-4675 (1977), describes the photooxidation of a number of organic and inorganic species, and suggests a method of synthesis involving indirect oxidation, e.g., photoelectrochemically oxidizing cerium(III) to cerium(IV), followed by reacting the cerium(IV) with a material, which simultaneously regenerates cerium(III).
It would be highly desirable to have available a method for photoelectrochemically generating the useful oxidant species thallium(III), which is then available for reaction with other materials.
Accordingly, it is an object of the present invention to provide a method for producing a solution of thallium(III) from thallium(I) in a photoelectrochemical cell.
It is a further object to provide a photoelectrochemical system for producing thallium(III) wherein the reaction requires no source of energy other than solar radiation.
A still further object of the invention is to provide a process for utilizing photoelectrochemically generated thallium(III) in reactions with organic compounds.
These, and other important objects, will become more apparent from consideration of the following description and examples.